The present invention relates to a controller for fluid pressure operated devices and, more particularly, to a controller capable of providing full system fluid flow to an auxiliary device when the primary device does not require the full flow.
Although the present invention is equally adaptable to any controller for fluid pressure operated devices wherein the controller utilizes a valve spool arrangement, it is especially advantageous when used in controllers for power steering systems of the type employed in off-the-road vehicles, and will be described in connection therewith. More specifically, although the invention will be described in connection with rotatable spool-sleeve valve arrangements, it will be appreciated that the invention may also be utilized with spool valves which operate in response to axial movement.
A controller for a power steering system of the type to which the present invention pertains is described in U.S. Reissue Pat. No. 25,126, assigned to the assignee of the present invention. Controllers of the type disclosed in the cited reissue patent have become well known in the art and generally comprise a housing having an inlet and an outlet and a pair of control fluid ports, feeding a power steering cylinder. The vehicle steering wheel is directly connected to the controller and when in the neutral (no input torque) condition, fluid may pass from the inlet through the valve to the outlet (open center system).
When the steering wheel is rotated in one direction from the neutral position, the valve is displaced and fluid flows from the inlet through the valve, to the meter, then to one of the control fluid ports to move the power steering cylinder. When the steering wheel is rotated in the opposite direction, the valve rotates in the opposite direction and fluid flows from the inlet port through the valve, then through the fluid meter in the opposite direction, then to the other of the control fluid ports to move the power steering cylinder in the opposite direction.
Conventionally, controllers of the type described have utilized rotary spool-sleeve valves to direct the flow of fluid from the inlet port in accordance with the rotational position of the steering wheel. In general, rotary spool-sleeve valves comprise a primary valve member (spool) connected directly to the steering wheel and a follow-up valve member (sleeve) surrounding the spool. Axially adjacent the spool and sleeve is a fluid meter, generally a gerotor having an externally toothed member coupled to orbit within an internally toothed member held in fixed position relative to the valve housing. The externally toothed member is splined to a drive shaft, at the opposite end of which the drive shaft is coupled to the sleeve, such as by a pin passing therethrough. When the spool is rotated, fluid is permitted to flow to the meter, causing the externally toothed member to orbit, thus imparting follow-up movement to the sleeve by means of the drive shaft. Generally, the sleeve has a plurality of orifices extending radially therethrough and the spool has a plurality of axially extending grooves on its outer surface to provide communication between certain of the orifices in the sleeve.
Power steering systems for off-the-road vehicles and the controllers used therein require expensive, complicated hydraulic pumping aparatus to provide the full system fluid flow capability required to actuate the power steering cylinder. Such off-the-road vehicles frequently utilize other hydraulically operated devices which also require approximately the full system fluid flow for their operation. Therefore, it has long been an objective of those working in the hydraulic power steering art to provide a system in which the full system fluid flow would be available to such an auxiliary device when not required by the steering system. Conventionally, the attempts at providing such a system have involved the use of a "load sensing" arrangement in which either a pressure sensing or flow sensing device has been included within the controller to sense the demand or lack of same for full flow by the steering system. This sensing device would, in turn, actuate a valve to divert or redirect the flow in response to a determination that the flow was not required by the steering system and was available for the auxiliary device. Other conventional systems apply a flow control valve with priority flow maintained by pressure compensation to supply a fixed flow rate to the primary (steering) circuit, with the excess (bypass) flow directed to a secondary circuit, thus requiring a larger pump to supply both circuits. With certain modifications, the prior art "high pressure carryover" or "power beyond" power steering systems have generally conformed to the above descriptions, thus requiring some sort of sensing device built into the controller or in fluid communication therewith, as well as additional valving and fluid flow conduits associated with the controller. Thus, the controllers for such high pressure carryover systems have been substantially more complicated and expensive than the basic controller and, in some instances, have had certain operational disadvantages, such as a transition from steering operation to auxiliary operation which is not smooth enough and/or too slow.